You need to cut 16x20 to 8x20 as well, right? Probably the first thing to do is borrow or rent a darkroom and cut all the paper you plan to use for this camera (the whole box, if you don't expect to enlarge that big from other sources -- you'll need paper to make prints from the negatives, too). With paper, you can do this under safelight conditions and actually see what you're doing.
With the paper pre-cut, you *might* be able to put the can on top of the box, close the bag, open and slide back the paper box lid just far enough to manipulate the paper out of the inner bag, close the box, work the paper into the can, and work the lid on. I'm not certain, however, on that. You'll need the largest size changing bag (about 27x30 inches, IIRC), or one of the "changing tent" or "dark box" setups (look at the local mini-lab to see a largish dark box -- they use them for loading rolls of paper into the magazine for their printing machines). Alternately, many folks find they can make a closet dark enough, especially for relatively slow paper; you'd go into the closet with your stuff, close the door, and do your changing.
Of course, then there's the issue of how you'll develop the negatives. Liquids inside a changing bag are a Very Bad Idea. Unless you're using the camera as a processing drum, you really, really need a darkroom (or else you're going to need 20 inches of 3" ABS pipe, a pair of caps, and the bits to put a light trap filler into one cap).
